Crystal and molecular structure of (±)-cyclomethylenomycinA

The structure of (±)-cyclomethylenomycinA, a penultimate synthetic precursor of the antibiotic methylenomycinA, was determined by direct methods from MoK diffractometer data. The crystals of (±)-cyclomethylen-omycinA are orthorhombic, space groupPbca, a=15.5724(1),b=11.4415(1),c=9.5454(1)Å, andZ=8. The structure was refined toR=0.077. The chemically determined structure was confirmed. Bond distances and angles are normal and there are no unusual C-H | | | O intermolecular contacts.IUPAC name: 3-(Hydroxymethyl)-1,5-dimethyl-4-oxo-6-oxabicyclo[3.1.0]hexane-2-carboxylic acid -lactone.     

Total synthesis of (±)-xanthocidin and (±)-desdihydroxy-4,5-dihydroxanthocidin

The first total synthesis of both (±)-xanthocidin ($\text{1}$), a novel α-methylene cyolopentanoid antibiotic, and (±)-desdihydroxy-4,5-dihydpoxanthocidin ($\text{2}$), the likely penultimate biosynthetic precursor is reported.     

Avermectin-milbemycin studies. 2. An efficient chemical degradation of avermectin b1a

An efficient chemical degradation of avermectin B_1a affording intact northern and southern hemispheres has been achieved.     

Crystal and molecular structure of reductiomycin

Conflicting results concerning the structure of the antitumor agent reductiomycin have been resolved. When the imino group and the heterocyclic oxygen atom of a previous x-ray study are interchanged, and this structure refined, it is found that: (i) a structure with satisfactory hydrogen bonding results, (ii) there are no unacceptably small nonbonded intermolecular distances, and (iii) agreement with two recent structure formulations based on various kinds of chemical and spectral evidence is achieved.Camille and Henry Dreyfus Teacher Scholar, 1978–1983; National Institutes of Health (National Cancer Institute) Career Development Awardee, 1980–1985.     

On the bernstein conjecture in approximation theory

WithE _2n (|x|) denoting the error of best uniform approximation to |x| by polynomials of degree at most 2n on the interval [?1, +1], the famous Russian mathematician S. Bernstein in 1914 established the existence of a positive constantβ for which lim 2nE _2n (|x|)=β.n→∞ Moreover, by means of numerical calculations, Bernstein determined, in the same paper, the following upper and lower bounds forβ: 0.278<β<0.286. Now, the average of these bounds is 0.282, which, as Bernstein noted as a “curious coincidence,” is very close to 1/(2√π)=0.2820947917... This observation has over the years become known as the Bernstein Conjecture: Isβ=1/(2√π)? We show here that the Bernstein conjecture isfalse. In addition, we determine rigorous upper and lower bounds forβ, and by means of the Richardson extrapolation procedure, estimateβ to approximately 50 decimal places.     

The spectrophotometric determination of uranium by the 1-pyrrolidinecarbodithioate method

Summary A spectrophotometric method for the determination of uranyl ion is proposed which is based on the use of ammonium 1-pyrrolidinecarbodithioate as the chromogenic reagent. Conformity to Beer's law was observed for the range between 0 to 50 ppm of uranium when spectrophotometric measurements are made at 385 nm.

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Zusammenfassung Ein spektrophotometrisches Verfahren zur Uranylbestimmung mit Ammonium-1-pyrrolidindithiocarbamat als Farbreagens wurde vorgeschlagen. Zwischen 0 und 50 ppm ist das Beersche Gesetz bei 385 nm erfüllt.

     

Dipole sums and intermolecular interaction coefficients derived from refractive index data

The old problem of using refractive index measurements to obtain estimates of dipole sums and interaction coefficients is reconsidered. It is found that a consistent numerical procedure for fitting the measured values to a two-term representation of the frequency dependent polarizability enables satisfactory values to be obtained.     

Synthesis and properties of butyl rubber–poly(ethylene oxide) graft copolymers with high PEO content

Butyl rubber‐poly(ethylene oxide) (PEO) graft copolymers with high PEO content (40–83 wt %) were synthesized by the functionalization and activation of the double bond moiety of butyl rubber containing high (7 mol %) isoprene content and subsequent reaction with PEO of different molecular weights from 750 to 5000 g/mol. The properties of these copolymers, along with other butyl rubber‐PEO graft copolymers were studied in films and in aqueous solution. Despite the high PEO content, films of the copolymers were quite stable in water with respect to mass loss and were capable of releasing an encapsulated probe molecule in a manner that was dependent on the PEO content. At high PEO content they were resistant to the adhesion and growth of C2C12 cells. Despite the resistance of films to dissolution, it was possible to prepare nanosized aqueous assemblies via a THF‐water exchange process and the sizes of the assemblies were tuned by their method of preparation. The assemblies were also able to encapsulate a probe molecule and were found to be nontoxic in vitro. Combined, this set of properties makes these new amphiphilic copolymers promising for a wide range of potential applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3383–3394